The present invention relates to a permanent magnet electric machine and, more particularly, to a permanent magnet electrical machine such as, for example, a motor or a generator, of a salient pole type which has a reduced cogging torque.
In electrical machines having salient magnetic poles, especially a brushless motor, a cogging torque occurs due to the salient pole structure and, for example in the U.S. Pat. No. 4,280,072, and Japanese Patent Publication No. 58-42708 an arrangement is proposed for reducing the cogging torque wherein auxiliary slots or grooves and grooves or slots for accommodating the windings are provided at equal intervals in surfaces of the salient magnetic poles, so as to render the degree number of harmonic components of the cogging torque higher, thereby reducing the cogging torque.
Generally, in addition to a cogging torque, a brushless motor also involves a ripple torque due to the fact that an induced voltage, i.e., a counter-electromotive force, of a brushless motor supplied by an inverter, as viewed from the input side of the inverter, the voltage has the same waveform as a full-wave rectification of an alternating current voltage which, as known, includes a ripple component causing a ripple in the torque produced by the brushless motor. Moreover, the ripple component is affected by the relationship between a pole pitch of a field and width of a salient pole of an armature. The pulsation rate of the cogging, in case of the providing of grooves or slots at equal intervals, becomes the same as at the least common multiple between the number of grooves or slots and the number of poles. Therefore, with the groove or slot arrangement at equal intervals, when three grooves or slots, including the winding groove or slot, are provided for one winding salient pole, in the case of three winding grooves among four poles, as described in, for example, the above noted U.S. patent, the circular measure of the magnetic pole is as small as 160.degree., and the cogging torque frequency is as low as thirty-six, so that the cogging torque is relatively high. With, for example, four grooves or slots, the cogging torque frequency is as low as twelve, and the cogging torque is great. With five or more grooves or slots, additional problems occur such as, for example, a loud wind noise due to the conspicuous lowering of magnetic flux magnitude attributed to the auxiliary groves or slots and the equal pitches of the grooves or slots.
The aim underlying the present invention essentially resides in providing a permanent magnet electric machine, such as a generator or motor, which is capable of substantially reducing the production of cogging torque.
More particularly, in accordance with advantageous features of the present invention, a permanent magnet electrical machine is provided which includes a permanent magnetic field member having a plurality of magnetic poles. An armature member is disposed in opposition to the field member and has a plurality of salient magnetic poles and armature windings wound to at least a portion of the salient magnetic poles. A greatest common division of the number of magnetic poles and number of salient magnetic poles wound by the armature winding differs from the number of magnetic poles. The armature member has at least one group of grooves or slots which are formed between neighboring or adjacent salient poles at electrical angular intervals of 2/3, 2/3.pi. with P being the number of pairs of magnetic poles of the field member which is not a multiple of three. The salient magnetic poles having auxiliary grooves on a surface opposite to the field member are separated from the grooves by an electric angle of (.pi./3)K+(.pi./6), with K being an integer.
Preferably, to reduce the cogging torque, one group of the grooves and the auxiliary grooves is shifted from another group, in terms of an electrical angle, by (.pi./12).multidot.L, with L being an odd integer.